1. Field of the Invention
The present invention relates to a resonator device, and more particularly, in a piezoelectric device provided with a vibrating element made of a piezoelectric material, the invention relates to a technology for suppressing fluctuations in resonant resistance depending on a driving signal level (hereinafter referred to as an excitation level) for exciting said device.
In comparison with the prior art resonant circuit, which consists of a combination of an inductance element, a capacitance element and so forth, the piezoelectric device can have a compact form and can operate with a smaller loss and with a stable temperature characteristic, so that it is suitably utilized in the field of audio and video-apparatus, TV sets, radio sets etc. as well as in a variety of electrical and electronic information and communication apparatus. Especially, the piezoelectric device is suitably used in an oscillator circuit, a band-pass filter etc., so that it is required to cover a wide dynamic range. In order to comply with this requirement, therefore, it must suppress fluctuations in the resonant resistance which cause oscillation failures, fluctuations in oscillation frequency and so forth, especially at a low operating voltage.
2. Description of the Related Art
A vibrating element as used in a piezoelectric device is formed by using a piezoelectric material such as lithium niobate (LiNbO.sub.3) or lithium tantalate (LiTaO.sub.3), and shows a resonant characteristic which is defined by its geometry. It is known well that such a resonant characteristic (i.e. resonant resistance value) very much depends on the excitation level for exciting the device.
FIG. 1a and FIG. 1b show graphically, the characteristics of fluctuation in loss level vs variation of excitation level for the piezoelectric device.
FIG. 1a shows fluctuation in resonator resistance (displayed in the form of loss level [dB]) when the excitation level is varied as indicated with circled numerals 1, 2, and 1 while FIG. 1b shows fluctuation in loss level when the excitation level is varied in an "acceptable" device, having no micro-cracks, which will be described later. In the figure, .DELTA.Rs represents a fluctuation (maximum) in loss level. The less it is, the more preferable. It can be seen from the examples shown in these figures that the device with the characteristic shown in FIG. 1b shows a smaller fluctuation .DELTA.Rs in loss level than the device with the characteristics shown in FIG. 1a.
As one of causes of such fluctuations in loss level, micro-cracks, which occur on the edge portions of the vibrating element as typically shown in FIG. 2, could be considered. In this figure, a reference numeral 21 designates a vibrating element, 22 a piezoelectric substrate, 23a (23a') and 23b (23b') a pair of driving electrodes, and 30 a micro-crack.
Such micro-cracks mainly occur during cutting in the production of the piezoelectric device (namely, in the course of cutting a wafer (piezoelectric substrate) having, on its main opposing surfaces, driving electrodes as formed in advance, thereby dividing the wafer into individual vibrating elements as a piezoelectric device). If, as shown in FIG. 2, the micro-cracks 30 occur on edge portions of the vibrating element 21, spurious modulation takes place and the concerned vibrating element shows characteristics like those which would occur if characteristics of severed vibrating elements were superimposed. Consequently, the resonator resistance i.e. loss level varies depending on a change in the excitation level.
Accordingly, the device in which fluctuations in loss level are within a predetermined allowable range, for example the device shown in FIG. 1b, may be shipped as a "acceptable" device, but the device of which the fluctuations (.DELTA.Rs) is large as shown in FIG. 1a, has to be rejected.
Heretofore, selection of the device, to determine whether to accept or reject the device, has been made by carrying out a test after the fabrication of the vibrating element. Such test is performed, for example, by the steps of varying the excitation level of the device at arbitrary intervals within a predetermined range, measuring the resonator resistance i.e. loss level at every excitation level, and evaluating the difference between maximum and minimum values of all the values measured (namely, maximum fluctuation quantity .DELTA.Rs as shown in FIGS. 1a and 1b).
In the prior art test method as described above, since the fluctuations in loss level of the piezoelectric device are evaluated by varying the excitation level of the device, it is possible to carry out the selection of the acceptable/rejected device, but this test method involves a problem in that the number of production steps is increased.
Also, a problem occurs in that a large fluctuation in the resonant resistance, i.e., loss level, depending on a variation in the excitation level of the device is caused by micro-crack generation as shown in FIG. 2, thereby causing oscillation failure, change of oscillation frequency and so forth occurring especially in the operation of the device at a low operating voltage. This is apparently undesirable from a stable operation viewpoint.
Further, in case of defining a test standard, since it is not possible to definitely specify the fluctuation quantity which causes oscillation failure, oscillation frequency change and so forth, there is caused inconvenience in that a test standard is made unnecessarily severe, for instance in that a device with fluctuation quantity .DELTA.Rs in its loss level of more than 1 dBm must be rejected. This undesirably results in a decrease in the yield rate of the piezoelectric devices.